1. Field of the Invention
The invention is directed to a piston-cylinder unit including a cylinder having an inner wall bounding a work space filled with a working medium, and a closure fixed axially with respect to the cylinder by position engagement with the cylinder. The closure bounds the work space and is sealed with respect to the wall by a circumferential seal, the closure being axially movable with respect to the cylinder when a temperature threshold is exceeded.
2. Description of the Related Art
It can not be ruled out in principle that a piston-cylinder unit is subject to extreme temperature loading during operation, for example, in case of an automobile accident in which the vibration damper of the motor vehicle is exposed to fire. The work medium found in the piston-cylinder unit undergoes a very great expansion, so that a push-out force acting on the piston rod and a closure overloads the fastening of the piston rod guide within the cylinder which is designed for normal operation. In theory, it may happen that the piston rod together with the piston is pushed out of the cylinder due to the pressure level within the piston-cylinder unit.
Solutions have already been suggested for this scenario. U.S. Pat. No. 5,454,455 discloses that at least one bead provides a stop surface for the piston rod-piston unit which moves out. This solution requires an adapted piston and in some cases leads to a loss of useful stroke because the bead is formed within the travel path of the piston.
U.S. Pat. No. 5,485,987 discloses several possibilities for reducing overpressure in a gas spring. For this purpose, the gas spring has a supporting body for the piston rod seal, which supporting body is constructed elastically in itself or is supported in a springing manner and releases a flow-out opening within an axial displacement path of the piston rod seal. However, there is also a solution in which the piston rod seal is destroyed by the increased operating pressure and an outlet channel is released. The possibilities shown in this reference require a supporting body whose inherent elasticity can be tuned to a defined temperature only with difficulty. A piston rod seal with a predetermined breaking point is at least appreciably more expensive than a conventional construction.
U.S. Pat. No. 4,988,081 discloses a safety fuse which releases a flow-out opening of the cylinder in case of fire. U.S. Pat. No. 5,064,030 is also to be mentioned in this connection. With respect to safety fuses, it can happen that the seal will lose its original contour and become mushy but will still continue to exert a sealing function.